Air tank and variable geometry air handling in hydrogen fuel cells

ABSTRACT

An air handling system for a fuel cell stack includes a pneumatic storage device disposed downstream from a compressor, a flow control valve system configured to operatively couple an inlet of the pneumatic storage device to an outlet of the compressor and configured to operatively couple an outlet of the pneumatic storage device to an inlet of the fuel cell stack, and a controller configured to, in response to a power demand being greater than a threshold, cause the flow control valve to open to increase a flow rate of air from the pneumatic storage device to the fuel cell stack.

TECHNICAL FIELD

The present disclosure generally relates to systems and methods foroperating a hydrogen fuel cell system.

BACKGROUND

Fuel cell systems, such as vehicles, use hydrogen or hydrogen-rich gasto power an electric motor. The fuel cell stack may generate electricityin the form of direct current (DC) from electro-chemical reactions thattake place in the fuel cells. A fuel processor converts fuel into a formusable by the fuel cell. If the system is powered by a hydrogen-rich,conventional fuel, such as methanol, gasoline, diesel, or gasified coal,a reformer may convert hydrocarbons into a gas mixture of hydrogen andcarbon compounds, or reformate. The reformate may then be converted tocarbon dioxide, purified and recirculated back into the fuel cell stack.

SUMMARY

An air handling system for a fuel cell stack includes a pneumaticstorage device disposed downstream from a compressor, a flow controlvalve system configured to operatively couple an inlet of the pneumaticstorage device to an outlet of the compressor and configured tooperatively couple an outlet of the pneumatic storage device to an inletof the fuel cell stack, and a controller configured to, in response to apower demand being greater than a threshold, cause the flow controlvalve to open to increase a flow rate of air from the pneumatic storagedevice to the fuel cell stack.

A method for operating an air handling system of a fuel cell stackincludes, in response to a power demand being greater than a threshold,controlling a flow control valve to increase a flow rate of air from apneumatic storage device to the fuel cell stack.

An air handling system for a fuel cell stack includes a turbineconfigured to recover energy from exhaust air output by the fuel cellstack, a compressor disposed downstream from the turbine and configuredto supply a first volume of air to the fuel cell stack, a pneumaticstorage device upstream of the fuel cell stack, and a controllerconfigured to, in response to a change in a power demand being greaterthan a threshold power demand, supply a second volume of air to the fuelcell stack from the pneumatic storage device, wherein the energyrecovered from the turbine is supplied to at least one of the turbine orthe controller.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description particularly refers to the following figures,in which:

FIG. 1 is a block diagram illustrating an example fuel cell system;

FIG. 2 is a block diagram illustrating an example air handling system ofthe fuel cell system of FIG. 1 ; and

FIG. 3 is a block diagram illustrating an example process flow foroperating the air handling system of FIG. 2 .

DETAILED DESCRIPTION

While the concepts of the present disclosure are susceptible to variousmodifications and alternative forms, specific exemplary embodiments arebeen shown by way of example in the drawings and will be described. Itshould be understood, however, that there is no intent to limit theconcepts of the present disclosure to the particular forms disclosed; onthe contrary, the intention is to cover all modifications, equivalents,and alternatives falling within the spirit and scope of the invention asdefined by the appended claims.

References in the specification to “one embodiment,” “an embodiment,”“an illustrative embodiment,” etc., indicate that the describedembodiment may include a particular feature, structure, orcharacteristic, but every embodiment may or may not necessarily includethat particular feature, structure, or characteristic. Moreover, suchphrases are not necessarily referring to the same embodiment. Further,when a particular feature, structure, or characteristic is described inconnection with an embodiment, it is submitted that it is within theknowledge of one skilled in the art to effect such feature, structure,or characteristic in connection with other embodiments whether or notexplicitly described. Additionally, it should be appreciated that itemsincluded in a list in the form of “at least one A, B, and C” can mean(A); (B); (C): (A and B); (B and C); (A and C); or (A, B, and C).Similarly, items listed in the form of “at least one of A, B, or C” canmean (A); (B); (C): (A and B); (B and C); (A and C); or (A, B, and C).

The disclosed embodiments may be implemented, in some cases, inhardware, firmware, software, or any combination thereof. The disclosedembodiments may also be implemented as instructions carried by or storedon one or more transitory or non-transitory machine-readable (e.g.,computer-readable) storage medium, which may be read and executed by oneor more processors. A machine-readable storage medium may be embodied asany storage device, mechanism, or other physical structure for storingor transmitting information in a form readable by a machine (e.g., avolatile or non-volatile memory, a media disc, or other media device).

In the drawings, some structural or method features may be shown inspecific arrangements and/or orderings. However, it should beappreciated that such specific arrangements and/or orderings may not berequired. Rather, in some embodiments, such features may be arranged ina different manner and/or order than shown in the illustrative figures.Additionally, the inclusion of a structural or method feature in aparticular figure is not meant to imply that such feature is required inall embodiments and, in some embodiments, may not be included or may becombined with other features.

A fuel cell is an electrochemical device that facilitates harnessingelectrical energy produced by a chemical reaction expressed usingEquation (1) such that:

$ H_{2} + \frac{1}{2}O_{2}arrow H_{2}O + 1é $

For the above reaction to take place, effective area of a single fuelcell (e.g., a cell) may be 1 cm² and each cell may produce a predefinedamount of voltage, e.g., 2.2 Volts (V). The current produced by a cellis directly proportional to its effective surface area. Fuel cells maybe connected together to form a fuel cell stack (e.g., stack assembly)that yields predefined levels of current and voltage.

To enable a continuous production of energy, a constant stream of oxygenand hydrogen need to be passed through the cell. Ideally, pure oxygenand hydrogen should be used as reactants. However, in the interest ofoperating economy, some systems or vehicles only store hydrogen on-boardand use ambient air as a source of oxygen.

Hydrogen is typically stored at very high pressure, e.g., 300-700 bar,in a specialized tank. The fuel cell stack assembly may be configured tooperate within a predefined temperature range, e.g., 60-80° C. (degreesCelsius). In some instances, performance of the fuel cell stack assemblymay deteriorate at higher operating temperatures, such as whentemperature of the fuel cell stack is greater than, for example, 90° C.Fuel cells use oxygen from the ambient air as an oxidizing agent toconvert chemical energy stored in a fuel storage tank into another form.

Fuel cells may exhibit a time delay (e.g., transient lag) in meeting adesired power demand value. In some instances, very fast transients(e.g., 150 kW/s in a 90-kW system) are more damaging to fuel cells andadvanced techniques may be necessary to limit the exposure of the fuelcells to transients. Durability of fuel cells may be affected by thenumber of times the fuel cells undergo a startup process. Accordingly,fuel cells may be used as steady state energy production devices and maybe coupled with batteries configured to handle the transient loads. Slowtransient operations also lead to the need for larger and more expensivebattery systems to ensure satisfactory battery life.

Controlling more accurately the pressure of air or oxidant introducedinto the air tank minimizes flow/pressure transients, thereby, improvingstack durability and reduces a time lag in air delivery to the stackshortening power ramp-up rate of the fuel cell stack. In some examples,the time lag in air delivery to the stack may be shortened from threeseconds or more to one second or less. Additionally or alternatively,optimizing tank pressure may modulate auxiliary load when power demandis below a predefined threshold avoiding unnecessary power down events.

In one example, a pressurized air storage device, such as an air tank,disposed downstream from a compressor of the air handling system mayquickly (e.g., one second or less) deliver flow of air to the fuel cellstack, thereby, decreasing or eliminating a time lag between thedelivery of fuel and the delivery of air to the fuel cell stack. Asanother example, positioning one or more pressurized air storage devicesdownstream from the compressor enables delivery of air to the fuel cellstack when the compressor is not operating, such as during shut downevents due to power drawn from the fuel cell stack being less than apredefined threshold. Each pressurized air storage device may be filledby the compressor via one or more flow control valves. As still anotherexample, a variable geometry turbine, e.g., instead of a fixed geometryturbine, disposed at (or proximate) the outlet of the fuel cell stackmay permit initiating turbine operation using the flow of air from thepressurized air storage device. Accordingly, implementation of the airstorage device and/or the variable geometry turbine within a given fuelcell system may enable achieving smoother flow and back pressurecontrol.

FIG. 1 illustrates an example fuel cell system 100 including a fuel cellstack 102, a power electronic controller 104 (e.g., system controller),a battery 106, a power electronics module 110, a motor-generator 112,and a transmission 114 (although, one or more of these variouscomponents such as power electronics controllers 104 can be utilized inthe system 100). The controller 104 monitors and controls operation ofthe fuel cell stack 102 and its various supporting subsystems. The powerelectronics module 110 includes one or more DC-DC converters configuredto electrically isolate operating voltage supplied to the fuel cellstack 102 from the battery 106 and bus voltage. The power electronicsmodule 110 includes an inverter and is configured to transfer powerbetween the battery 106 and the electric motor-generator 112 connectedto the transmission 114.

The fuel cell stack 102 includes a plurality of cells (e.g., fuel cells)within which an electro-chemical reaction takes place to generateelectric current. Fuel, such as hydrogen or a hydrocarbon, is channeledthrough field flow plates to the anode on one side of the fuel cell,while oxygen from the air is channeled to the cathode on the other sideof the cell. At the anode, a catalyst, such as a platinum catalyst,causes the hydrogen to split into positive hydrogen ions (protons) andnegatively charged electrons. The polymer electrolyte membrane (PEM)permits the positively charged ions to flow through the PEM to thecathode. The negatively charged electrons are directed along an externalloop to the cathode, creating an electrical circuit (electricalcurrent). At the cathode, the electrons and positively charged hydrogenions combine with oxygen to form water, which flows out of the fuelcell.

Generally, the amount of power produced by each fuel cell of the fuelcell stack 102 at least partly depends on the amounts of reactantspassing through the fuel cell. As described in reference to at leastFIG. 2 , the fuel handling and delivery loop operates under a relativelyconstant pressure (e.g., fluctuation within 1 bar of a predefinedpressure value), which ensures that the fuel is delivered to the fuelcell nearly instantaneously following an increase in demanded power. Anair handling system of the fuel cell vehicle, system or device pressuresmay fluctuate more significantly (e.g., fluctuation of more than 1 bar,fluctuation of 2 bar or more, fluctuation of 3 bar or more, and so on)during normal operation, as such the flow of air required for meeting agiven power demand or a change in a power demand may be unavailable ormay be delivered to the fuel cell after a delay. Moreover, since ambientair contains only about one-fifth (or 20%) oxygen by volume, the amountof air that needs to be circulated through the system is at least twotimes the amount of hydrogen. Accordingly, the delivery of a sufficientamount of air to the fuel cell 102 may be slower, than the delivery offuel, causing a delay in power generation. Such delayed production ofpower causes high current draw on the battery 106, thereby, negativelyimpacting battery 106 longevity.

Moreover, during any delay in power production, hydrogen being passedthrough the system is ultimately wasted. An increased power demand thatis unmet, even for a short period of time, causes a pressure imbalancebetween anode and cathode plates bringing about airflow transients thatdegrade operation of the fuel cell. As one example, the membrane of thePEM fuel cell is particularly vulnerable to transients resulting fromflow and pressure fluctuations. Such transients may damage or otherwiseshorten a useful life span of the membrane.

FIG. 2 illustrates an example fuel cell power system 200 including anair handling system 220 configured to provide improved conditioning ofair and minimize pressure fluctuations at an inlet 242 of the fuel cellstack 202. In particular, the air handling system 220 includes apneumatic storage device 204, such as an air tank, and a plurality offlow control valves 206, 208, 210.

The air handling system 220 supplies regulated air to the fuel cellstack 202 to feed the power generation reaction. Ambient air enteringthe air handling system 220 may pass through an air filter 218 beforeentering a compressor 222. A motor 224 controls the speed of thecompressor 222 to both increase the air pressure and set the air flowrate. A controller, such as the system controller 104, can controloperation of the motor 224. The cooled air output by the compressor 222passes through an air humidifier 226 configured to saturate the air flowwith water at the operating temperature of the stack 202, which can bemonitored and controlled such as through a controller. A heat exchanger228 receives the saturated air output by the air humidifier 226.

The pneumatic storage device 204 is configured to ensure that a steadystream of fresh air is available during increased power demands. In oneexample, in response to power demand value being greater than apredefined threshold, the pneumatic storage device 204 may be configuredto supply a flow of air into the air handling system 220 of the fuelcell system 200. As another example, the pneumatic storage device 204may be configured to supply a flow of air into the fuel cell stack 202in response to a change in power demand value being greater than apredefined threshold. In still another example, the pneumatic storagedevice 204 may be configured to provide a flow of air into the airhandling system 220 in response to some combination of a power demandvalue and a threshold change in a power demand value.

The pneumatic storage device 204 may be operably coupled to the airhumidifier 226 via a first flow control valve 206. The pressurized airin the device 204 can be used as source of fresh air to begin powerproduction. In one example, the pneumatic storage device 204 may beconfigured to direct a flow of air into the aid humidifier 226 when thefist flow control valve 206 is open and prevent a flow of air into theaid humidifier 226, from the pneumatic storage device 204, when thefirst flow control valve 206 is closed. In one example, operation of thefirst flow control valve 206 may be monitored and controlled by thesystem controller 104 of FIG. 1 . The pneumatic storage device 204 maybe filled via a second flow control valve 208 that operatively connectsthe storage device 204 to a line at an outlet 252 of the compressor 222.As such, an onboard air storage device, such as the pneumatic storagedevice 204, ensures availability of a larger amount of air within agiven time period to minimize delays in achieving an effectiveelectro-chemical reaction. In a manner similar to that described abovefor the first flow control valve 206, the system controller 104 may beconfigured to monitor and control operation of the second flow controlvalve 208 and/or any other control valve disclosed herein.

Moreover, to prevent the fuel cell stack 202 from shutting down inresponse to power drawn being less than a predefined amount, whichimpacts durability and useful life of the fuel cell stack 202, thecompressor 222 may be configured to act as an auxiliary load to chargethe pneumatic storage device 204. Thus, the storage device andcompressor are configurable to prevent fuel cell shut down in responseto certain operating conditions. When the fuel cell stack 202 is turnedoff, pressurized flow from pneumatic storage device 204 may be used toinitiate the delivery of air into the fuel cell stack 202, thereby,jumpstarting power production reaction, without having to turn on thecompressor 222.

An air condenser 214 and a turbine 216 handle the exhaust air streamexiting the fuel cell stack 202. A pressure control valve 212operatively connecting an outlet 244 of the fuel cell stack 202 to aninlet 246 to the air condenser 214 operates to remove water and watervapor from the exhaust air output by the cathode of the fuel cell stack202. The air condenser 214 cools the exhaust air stream, at which pointthe air stream enters an inlet 248 of the turbine 216. The turbine 216converts the thermal and flow (pressure) energy of the air stream comingfrom the fuel cell stack 202 into mechanical energy. This recoveredmechanical energy may assist an electric machine, such as the motor 224,in running the compressor 222. Air exiting the turbine 216 is releasedinto the atmosphere.

In an example, the pneumatic storage device 204 may be operably coupledto a line extending between an outlet 250 of the air condenser 214 andthe inlet 248 of the turbine 216 via a third flow control valve 210. Ina manner similar to that of the first flow control valve 206, thepneumatic storage device 204 may be configured to direct a flow of airinto the line downstream of the outlet 250 of the air condenser 214 whenthe third flow control valve 210 is open and prevent a flow of air intothe line at the outlet 250 of the air condenser 214, from the pneumaticstorage device 204, when the third flow control valve 210 is closed.Other methods for operating the third flow control valve 210 are alsocontemplated. In an example, the third flow control valve 210 may beconfigured to permit the flow of air from the pneumatic storage device204 while preventing the flow of air from the air condenser 214.

In some instances, such as during startup events, the pneumatic device204 may be configured to direct the flow of air into the turbine 216,such as through the third flow control valve 210. In some instances, thefuel cell system 100, 200 may be equipped with a variable geometryturbine such that the flow of air from the pneumatic device 204 causestorque to be applied onto the compressor 222, thereby initiatingdelivery of fresh air to the cathode of one or more fuel cells of thefuel cell stack 202. This approach causes a presence of hot air at aninlet 242 to the cathode side of the fuel cell stack 202, thereby aidingduring cold start and warmup events. Geometry of the variable geometryturbine may be adapted in accordance to operating conditions to modulatecompressor speed, load and/or pressure ratio between the air handlingsystem and the atmosphere. Moreover, for fuel cell systems that operateat higher pressures, a turbine may be coupled directly to the outlet ofthe cathode to increase an amount of energy recovered from the cathodeexhaust gas.

Accordingly, the air handling system 220 of the present disclosureminimizes a delay in delivering oxygen necessary for the power producingreaction to take place. The air handling system 220 is configured toensure a steady and constant availability of oxygen to each fuel cell ofthe fuel cell stack 202, thereby, minimizing the transients and increaseoperating life of the fuel cell(s) and the fuel cell stack 202.

A fuel handling system 240 of the fuel cell power system 200 includesone or more hydrogen storage tanks 230. A recirculation pump 232 isconfigured to selectively draw at least a portion of the hydrogen fuelfrom the one or more hydrogen storage tanks 230 and direct the portionof hydrogen toward the anode side of the fuel cell stack 202. Anelectrically and/or mechanically controlled pressure valve 234 disposedbetween an inlet 254 of the recirculation pump 232 and an outlet 256 ofthe anode of the fuel cell stack 202 may be configured to selectivelyrelieve pressure by releasing excess hydrogen within the fuelrecirculation loop. At least a portion of anode exhaust gases at theoutlet 256 of the anode of the fuel cell stack 202 may be directed tothe recirculation pump 232 where the exhaust gas may be mixed withhydrogen fuel from the storage tanks 230.

A reaction that combines hydrogen and oxygen to produce water andelectricity is an exothermic reaction and generates heat. A thermalsystem including stack cooling 238 of the fuel cell vehicle isconfigured to reject the heat and maintain operating temperature of thefuel cell stack 202 within a predefined threshold or temperature range.

The fuel cell stack 202 generates energy (e.g., electric power) to powera powertrain 250, which can be integrated with the fuel cell system 200or a separate system of the vehicle. The powertrain 250 may include oneor more of a drive motor, e.g., the motor-generator 112, a transmission,e.g., the transmission 114, and/or one or more ancillary components. Thefuel cell stack 202 energy may be used to maintain charge of anauxiliary vehicle battery of a battery and electronics 236, e.g., a lowvoltage battery supplying power to one or more components or devicesconnected to a low voltage bus. The auxiliary vehicle battery of thebattery and electronics system 236 may operate in a manner similar tothat of the battery 106 described in reference to FIG. 1 . Additionallyor alternatively, at least a portion of the power generated by the fuelcell stack 202 may be transferred directly to a plurality of electronicdevices onboard the fuel cell vehicle, such as, but not limited to, oneor more electronic controllers or devices configured to condition outputpower of the fuel cell stack 202, e.g., one or more components of thepower electronics module 110 described in reference to FIG. 1 , one ormore system controllers, e.g., system controller 104 of FIG. 1 , one ormore controllers or other components of a steering system, propulsioncomponents cooling system and interior cabin heating ventilation and A/C(HVAC) system, and so on.

FIG. 3 illustrates an example process flow 300 for operating an airhandling system, such as the air handling system 200 of FIG. 2 . Theprocess 300 may be executed by one or more controllers of the fuel cellsystem, the vehicle, or some combination thereof. In one example, thesystem controller 104 executes process 300 in response to acorresponding parameter threshold being met or in response to acorresponding command. The process 300 begins at block 302 where thecontroller 104 receives a signal indicating a present power demandparameter value. While the process 300 describes monitoring andevaluating a power demand parameter value, in some instances, thecontroller 104 may receive a signal indicating a value of a differentparameter, such as a change in power demand parameter value.

At block 304, the controller 104 determines whether a received powerdemand value is greater than a predefined threshold. In response to thereceived power demand value being less than a predefined threshold, acontroller such as the controller 104, at block 306, prevents flow ofair from the pneumatic storage device 204 into the fuel cell system 200.The controller 104 may then end or exit the process 300.

In response to power demand value and/or a change in power demand valuebeing greater than a predefined threshold, a controller such as thecontroller 104, at block 308, initiates (or continues) a flow of airfrom the pneumatic storage device 204. At block 310, the controller 104determines whether a requested power demand has been met. The controller104 returns to block 308 where it continues to cause a flow of air fromthe pneumatic storage device 204, in response to the power demand notbeing met. At block 312 the controller 104, in response to the powerdemand being met, stops the flow of air from the pneumatic storagedevice 204 into the fuel cell system 200.

The presence of reserve pressurized air, in accordance with the presentdisclosure, enables the air handling system to respond quicker to suddenincrease in air demand to thereby improve the system’s transientperformance. Intelligent actuation of output from the tank canselectively increase a pressure level in the air handling system to adesired pressure level and/or maintain a pressure level in the airhandling system at a predefined desirable pressure level, which willminimize flow transients and improve durability of the fuel cell stack.Intelligent actuation of output from the tank enables the fuel cell torespond quicker (e.g., within 3-6 seconds from zero power to maximumpower value) to an increase in power demand, thereby improving transientpower response and shortening a time period to achieve peak power.

While the disclosure has been illustrated and described in detail in thedrawings and foregoing description, such an illustration and descriptionis to be considered as exemplary and not restrictive in character, itbeing understood that only illustrative embodiments have been shown anddescribed and that all changes and modifications that come within thespirit of the disclosure are desired to be protected.

There are advantages of the present disclosure arising from the variousfeatures of the method, apparatus, and system described herein. It willbe noted that alternative embodiments of the method, apparatus, andsystem of the present disclosure may not include all of the featuresdescribed yet still benefit from at least some of the advantages of suchfeatures. Those of ordinary skill in the art may readily devise theirown implementations of the method, apparatus, and system thatincorporate one or more of the features of the present invention andfall within the spirit and scope of the present disclosure as defined bythe appended claims.

1-20. (canceled)
 21. An air handling system for a fuel cell stack, thesystem comprising: a compressor; a pneumatic storage device disposeddownstream from the compressor; an air humidifier disposed downstreamfrom the compressor and the pneumatic storage device; a flow controlvalve system configured to operatively couple an outlet of the pneumaticstorage device to the air humidifier and an inlet of the fuel cell stackand to operatively couple an outlet of the compressor to an inlet of thepneumatic storage device and the air humidifier; and a controllerconfigured to, in response to a power demand being greater than athreshold, cause at least a portion of the flow control valve system toopen to increase a flow rate of air from the pneumatic storage device tothe fuel cell stack through the air humidifier.
 22. The system of claim21, wherein the flow control valve system includes a first flow controlvalve disposed in a first line and configured to operatively couple theoutlet of the pneumatic storage device to the air humidifier and theinlet of the fuel cell stack.
 23. The system of claim 22, wherein theflow control valve system includes a second flow control valve disposedin a second line and configured to operatively couple the outlet of thecompressor to the inlet of the pneumatic storage device and the airhumidifier.
 24. The system of claim 23, wherein the controller isconfigured to, in response to the power demand being greater than thethreshold, cause the first flow control valve to open to increase theflow rate of air from the pneumatic storage device to the fuel cellstack through the air humidifier or is configured to cause the secondflow control valve to open to transfer air from the compressor to thepneumatic storage device to increase an amount of air stored in thepneumatic storage device.
 25. The system of claim 21, wherein thecompressor is configured to supply a first volume of air to the fuelcell stack through the air humidifier.
 26. The system of claim 25,wherein the pneumatic storage device is configured to supply a secondvolume of air to the fuel cell stack through the air humidifier.
 27. Thesystem of claim 26, wherein the controller is configured to stopoperation of the compressor in response to the supply of the secondvolume of air from the pneumatic storage device to the fuel cell stack.28. The system of claim 26, wherein the compressor is configured toselectively supply a third volume of air to the pneumatic storagedevice.
 29. The system of claim 28, wherein the air humidifier isconfigured to mix the first and second volumes of air upstream of thefuel cell stack.
 30. The system of claim 21, further comprising avariable geometry turbine operatively coupled to the compressor anddisposed downstream from the fuel cell stack, wherein the variablegeometry turbine is configured to recover energy from a direct airstream exhausted from the fuel cell stack.
 31. An air handling systemfor a fuel cell stack, the system comprising: a compressor; a variablegeometry turbine operatively coupled to the compressor and configured torecover energy from a direct air stream exhausted from the fuel cellstack; a flow control valve system configured to operatively couple anoutlet of the compressor to an inlet of the fuel cell stack and tocouple an outlet of the fuel cell stack to an inlet of the variablegeometry turbine; and a controller configured to, in response to a powerdemand being greater than a threshold, cause at least a portion of theflow control valve system to open to increase a flow rate of air fromthe compressor to the fuel cell stack.
 32. The system of claim 31,wherein the variable geometry turbine is configured to modulate at leastone of an operating speed of the compressor, a power drawn from the fuelcell stack, or a pressure ratio.
 33. The system of claim 31, wherein theflow control valve system comprises: a first flow control valve disposedin a first line operatively coupling the outlet of the compressor to theinlet of the fuel cell stack; and a second flow control valve disposedin a second line operatively coupling the outlet of the fuel cell stackto the inlet of the variable geometry turbine.
 34. The system of claim33, wherein the flow control valve system further comprises a third flowcontrol valve disposed in the second line and disposed downstream fromthe second flow control valve, the third flow control valve beingoperatively coupled to the inlet of the variable geometry turbine, andwherein the controller is configured to selectively cause the third flowcontrol valve to open to increase a flow rate of air into the inlet ofthe variable geometry turbine such that torque is applied onto thecompressor and a flow of air is delivered to the fuel cell stack. 35.The system of claim 34, further comprising a pneumatic storage devicedisposed downstream from the compressor and upstream from the fuel cellstack.
 36. The system of claim 35, wherein the third flow control valveoperatively couples an outlet of the pneumatic storage device to theinlet of the variable geometry turbine, and wherein the controller isconfigured to selectively cause the third flow control valve to open toincrease the flow rate of air from the pneumatic storage device into theinlet of the variable geometry turbine.
 37. The system of claim 31,further comprising an air condenser disposed downstream of the fuel cellstack and upstream of the variable geometry turbine.
 38. The system ofclaim 37, wherein the flow control valve system comprises a valve andthe valve operatively couples the outlet of the fuel cell stack to aninlet of the air condenser.
 39. The system of claim 38, wherein thedirect air stream exhausted from the fuel cell stack passes through thevalve when the valve is open, through the air condenser, and enters thevariable geometry turbine.
 40. The system of claim 31, furthercomprising a pneumatic storage device disposed downstream from thecompressor and upstream from the fuel cell stack.